1. Field of Invention
The present invention relates to a lithographic apparatus and a method for controlling a thermal optical manipulator.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the projection system of such lithographic apparatus, or in general in any optical system, aberrations may occur due to e.g. thermal effects in optical elements. As an example, a lens of the projection system may be subjected to a thermal profile, which may lead to differences in refractive index of a material of the lens, as the refractive index may show a temperature dependency. Thermal differences in optical elements may occur due to various causes: absorption of some of the radiation may cause an increase in temperature, gas flows may lead to a local increase or decrease of a surface temperature of an optical element, etc. To be able to at least partly compensate for such aberrations, a thermal optical element has been devised, as disclosed in co-pending U.S. patent application Ser. No. 11/593,648, which is incorporated herein in its entirety by reference.
Such thermal optical element is provided with a matrix of heater elements arranged on a surface thereof, each heater element e.g. consisting of a layer of conductive material and being able to be provided with electrical power to heat the heater element. The heater elements may thereby form a matrix, each of the heater elements may be provided with a suitable amount of electrical power to generate a suitable amount of heat thereby bringing the respective part of the thermal optical element to a suitable temperature.
Controlling a heating of each of the heater elements of the matrix may lead to a highly complex control structure. Furthermore, many parasitic effects would have to be taken into account. For example, conductors toward each of the heater elements of the matrix will provide for an amount of electrical dissipation in other parts of the thermal optical element.